# The value of cos

We have, cos 12° + cos 84° + cos 156° + cos 132°

= (cos 132°+ cos 12°) + (cos 156° + cos 84°)

= 2 cos72°.cos60° + 2 cos120°.cos36°

= cos72° - cos36°

= cos(90° -18°) – cos36°

= sin 18° - cos36°

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